Cambodian government troops opened fire on striking garment workers demonstrating for higher wages Friday, killing at least four and drawing condemnation from the United States and human rights groups.

The violence in Phnom Penh's Veng Sreng industrial district was described by Asian news media on the scene as the worst in the country in 15 years.

Reports from the city also described the garment workers' strike and alliance with opposition political forces as the most significant challenge to Prime Minister Hun Sen in his nearly three decades in power.

The garment industry is vital to the Cambodian economy, employing about 600,000 people and providing 80% of the country's exports, worth $5 billion annually. But most factories have been closed for the last 10 days while workers demanding an eventual doubling of their $80 monthly wages have been on strike.

Police shot directly into crowds of demonstrators, Chan Soveth, lead investigator for the rights group Adhoc, told Voice of America Cambodia.

Conflicting casualty tolls were reported, with most news agencies in Phnom Penh putting the number of dead at four and the wounded as high as 40. The newspaper the Australian said there were five fatalities in the melee, the second violent clash between strikers and state security in as many days.

Cambodian rights groups and foreign governments were quick to denounce the violence and call for restraint on all sides.

"We condemn this appalling use of extreme lethal force by security forces," Naly Pilorge, director of the Cambodian League for the Promotion and Defense of Human Rights (LICADHO), said in a statement demanding an immediate end to the use of live ammunition.

"The United States deeply regrets the recent loss of life in Cambodia during violent clashes between protesters and government security forces," State Department spokeswoman Marie Harf said in Washington. "We condemn violence as a means to achieve political or other objectives and urge all sides to exercise maximum restraint and show respect for the rule of law."

Cambodian opposition leader Sam Rainsy's Cambodia National Rescue Party has been staging protests against Hun Sen's Cambodian People's Party since July, claiming fraud gave the CPP its parliamentary majority and demanding a new vote.

The opposition has gotten a major boost from the garment workers' strike, with the combined protest forces putting tens of thousands on the streets in the most significant challenge of Hun Sen since he came to power in 1985.